Optical communication systems are a substantial and fast-growing constituent of communication networks. The expression "optical communication system," as used herein, relates to any system that uses optical signals to convey information across an optical waveguiding medium, such as an optical fiber. Such optical communication systems include, but are not limited to, wavelength division multiplexed ("WDM") telecommunications systems, cable television systems, and local area networks (LANs).
To convey information from multiple sources, time-division multiplexing (TDM) is frequently employed. In TDM a particular time slot is assigned to each information source, the complete signal being constructed from the signal portions collected from each time slot. Therefore, while this is a useful technique for carrying plural information sources on a single channel, fiber dispersion and other optical effects limit its capacity. While the need for communication services increases, the current capacity of installed waveguiding media is limited. Although capacity may be expanded, e.g., by laying more fiber-optic cables, such expansion is expensive.
WDM increases the capacity of installed fiber optic networks by transmitting multiple optical signal channels over a single optical fiber, each channel being assigned a particular channel wavelength. Those skilled in the art appreciate that the wavelength of an optical signal in a particular medium is related to a frequency, and that either designation may be used to describe a signal. In a typical WDM system, optical signal channels are: (1) generated; (2) multiplexed to form an optical signal constructed of the individual optical signal channels; (3) transmitted over a single waveguide; and (4) demultiplexed such that each channel wavelength is individually routed to a designated receiver. To maintain sufficient power in each optical channel, optical amplifiers, such as erbium-doped fiber amplifiers, can amplify the optical signal channels. This facilitates the use of WDM systems in trunk lines of various optical communication systems. Exemplary WDM optical communication systems are described in U.S. Pat. Nos. 5,504,609; 5,532,864; and 557,442.
In many applications, such as optical LANs, cable television subscriber systems, and telecommunications networks, there is a need to selectively change the route of one or more channels of a multiplexed optical signal from one destination to another. Such routing occurs when optical channels are sent to or received from an optical transmission line e.g., for sending of the optical channels between a terminal and an optical bus or routing long distance telecommunications traffic to individual cities.
To fully exploit the increased capacity of WDM technology, a method of routing is needed so that optical signals arriving at a single point, or node, from other locations can be efficiently routed to a selected destination. Preferably, such routing occurs without converting from optical signals to electrical signals, and should be applicable to either time domain multiplexing or re-conversion to optical signals and re-transmission. An ideal routing system is efficient, reliable, low cost, and responsive to the demands of optical signal channels arriving at the nodes.
In order to route optical signal channels to and from sources and destinations, the optical signal channels must be combined and separated, such as with wavelength-dependent multiplexers and de-multiplexers. A WDM multiplexer receives signals at different wavelengths from many sources and combines them into a single output signal. The channels are generally assigned a nominal wavelength, and the channel spacing, or separation, (expressed as wavelength or frequency) between channels is typically defined for the network. The closer the channel spacing, the greater the number of channels that may be transmitted over an optical fiber of the network. The channel spacing is limited by a number of factors, such as the stability of the light source, which is often a distributed feedback ("DFB") laser. Typically, external modulation of the light from the light source forms the encoded optical signal channel having the required information. DFB lasers have an inherently narrow line width, typically less than 1 GHz. However the optical signal channels must be spaced a sufficient distance to prevent overlap arising from signal drift.
DFB lasers, unfortunately, suffer from both steady state drift over their lifetime as well as thermal drift. The combination of both thermal and steady state drift, coupled with allowances for manufacturing variances, results in a performance envelope with a width of up to approximately .+-.20 GHz from a nominal wavelength position. The total envelope width, therefore, is about 40 GHz.
Recently the International Telecommunications Union has proposed a Dense WDM ("DWDM") network standard with optical signal channels having a frequency separation of 100 GHz (equivalent to a wavelength separation of about 0.8 nm). As such, the performance requirements for a DWDM network (such as those for bandwidth, cross talk, polarization dependent loss, polarization mode dispersion, insertion loss) are becoming more stringent. Additionally, it is anticipated that future efforts to increase capacity by reducing optical channel spacing will require additional improvements in optical system components, such as transmitters, receivers, multiplexers, and demultiplexers. Currently, conventional WDM multiplexer and de-multiplexer devices suffer from many performance deficiencies and consume a large portion of an optical system's attenuation loss budget. As such, any future multiplexer or de-multiplexer should be manufactured inexpensively, while also being small in size for easy installation, reliable, and have stable performance over a variety of environmental conditions.
In addition to the above problems, selective routing is difficult in a DWDM communication system because of the limitations introduced by conventional optical switches. Typically, conventional optical switches switch all incident wavelengths at once. To dynamically vary the selective routing of an optical signal channel, therefore, all signal channels are demultiplexed into signals of different wavelengths. Once the optical signals are demultiplexed, various types of electro-optical switches may be activated to redirect each signal toward its destination. Unfortunately, to effectively utilize the total bandwidth of each route, the signals must be multiplexed again, after they are switched. Thus, each potential route requires a demultiplexer, multiplexer, and optical switches for each optical channel. This combination of multiplexers and demultiplexers in series, results in signal loss from a phenomenon known as bandwidth narrowing. Additional, signal losses (such as insertion losses, polarization mode dispersion and ripple) from each combination of multiplexer/demultiplexer will accumulate, thus imposing cost penalties, such as the need for more frequent or more powerful amplification, and/or limits on the interconnection density in the network.
For the foregoing reasons, there is a need for an optical switch which can select, that is drop or add, one or more optical channels by wavelength de-multiplexing/multiplexing only those selected optical channels, with minimal attenuation or bandwidth narrowing of the other, non-selected, optical signal channels or otherwise effecting the non-selected optical channels. Additionally, there is a need for an optical switch that is small in size, highly reliable, and has only a single-point failure mode (i.e. an actuation failure affects only a single WDM channel), low insertion loss and adjacent channel tilt on all channels, and is easily interconnected or cascaded.